The present invention is directed generally to extrusion-formed articles of uranium-2.4 wt.% niobium alloys with controlled linear thermal expansion and, more particularly, to the preparation of such articles in which the linear thermal expansion in the direction transverse to the longitudinal axis of the extrusion is less than 0.98% over a temperature range of 22.degree. C. to 600.degree. C.
The formation of uranium-2.4 wt.% niobium alloy articles by the alphaphase extrusion of the alloy has been used to provide high strength and yet ductile articles of the alloy which have a yield strength at 0.2% offset of at least 400 MPa, an ultimate tensile strength of at least 1,050 Mpa, a compressive yield strength at 0.2% offset of 675 MPa and an elongation greater than 25%. These extruded uranium alloy articles have a polycrystalline structure which provides a linear thermal expansion (LTE) in the direction transverse to the longitudinal axis of the extrusion of greater than 1.04% in a temperature range between 22.degree. C. and 600.degree. C.
While the strength and ductility of the extruded alloy provide desirable properties, the LTE in the direction transverse to the direction of extrusion may be excessive in applications when the article is associated with another material having a linear expansion of less than the U-2.4 Nb alloy. For example, in applications where an article of the uranium alloy is clad or provided with a protective coating and subjected to temperature variations within the aforementioned range, the LTE in the transverse direction may cause stresses or the separation of the coating or cladding from the uranium article due to the differences in the expansion of alloy and the material used for the cladding or coating. Also, in applications where the alloy article is brazed to a support member, the brazing temperatures necessary for effecting the joint may heat the alloy article sufficiently to cause a strain at the brazed joint upon cooling due to the LTE of the alloy so as to fail or deleteriously weaken the brazed joint.
As will be described below, the aim of the present invention is to prepare articles of the uranium-2.4 wt.% niobium alloy by extrusion that possess the aforementioned strength and ductility properties of the alloy and which also posess a crystallographic texture that provides an LTE in the transverse direction of less than 0.98% between the the temperatures of 22.degree. C. to 600.degree. C. The thermal expansion of uranium-2.4 wt.% niobium alloy is dependent upon both the phase changes during the measurement of thermal expansion and the crystallography of the alpha phase in the direction of measurement. The thermal expansion behavior is dependent upon the crystallography due to the anisotropic nature of the thermal expansion of uranium. The mean axial thermal expansion coefficients of alpha uranium are +35.6 and +31.6.times.10.sup.-6 /.degree.C. in the a and c axes, respectively, and -8.4.times.10.sup.-6 /.degree.C. for the b axis between 27.degree. C. and 640.degree. C. As described in the article, "Preferred Orientation in Extruded Uranium Rod", reported in Transactions of the AIME, Journal of Metals, February 1957, p. 291, the crystallographic texture of alpha uranium is changed by the alpha-phase extrusion. During extrusion operations the b crystallographic axis of the alpha phase in the uranium body being extruded is rotated in a direction transverse to the direction of extrusion if the uranium is being extruded in the alpha phase. Inasmuch as the LTE of the b axis of the alpha phase of uranium is negative, the net LTE of the polycrystalline uranium should be lowered in a direction transverse to the direction of extrusion.
However, efforts to provide an extruded uranium-niobium article with desired mechanical properties as described above while manipulating the b axis for providing a decrease in the LTE by using known alpha-phase extrusion practices proved unfruitful. For example, using a uranium-niobium billet preheat temperature of 590.degree. C. was expected to assure that a billet temperature within the alpha-phase temperature region would be provided during the extrusion operation. However, while the extrusion in the alpha phase was successful, some central bursting defects occurred within the extruded alloy article so as to destroy the soundness of the alloy, thereby rendering it useless. To obviate the central bursting defects, a billet preheat temperature of 630.degree. C. was utilized, but this preheat temperature caused the temperature of the billet to increase above the alpha-phase temperature regions (663.degree. C.) during the extrusion and, in effect, destroyed the desirable alpha-phase crystallographic texture without decreasing the LTE to desired levels.